An optical member is utilized in a wide range of fields such as a camera and a telescope. The optical member can be roughly classified into two types using a crystal as a starting material and glass as the starting material. A crystalline optical member of these optical members is divided in use depending on a crystal system. An optical member used as a lens in an imaging optical system is formed of a cubic crystal. By using an optically isotropic cubic crystal, it is possible to reduce birefringence or the like due to optical anisotropy.
With high integration of a semiconductor integrated circuit, demands on ultrafine pattern formation have increasingly grown. A reduced projection exposure device (stepper) of a step-and-repeat type for transferring a minute pattern onto a wafer grows more sophisticated, so that a wavelength of a light source for exposure is shifted to a short wavelength. An optical member which receives attention in optical members for that purpose is a cubic calcium fluoride single crystal having a high transmittance in an ultraviolet region. Further, in recent years, development of optical members using Lu, Al, Mg, and the like higher in refractivity than Si has been tried in order to realize a high refractive index of an optical member for the purpose of providing a higher resolution. For example, development of cubic crystals such as lutetium aluminum garnet single crystal (Lu3Al5O12), magnesium oxide single crystal (MgO), and magnesium spinel single crystal (MgAl2O4) have been actively carried out. Particularly, the lutetium aluminum garnet single crystal has a high refractive index, thus being expected for future development. For example, the lutetium aluminum garnet single crystal has a refractive index of 2.1 at a wavelength of 193 nm. Quartz glass has a refractive index of 1.56 at the wavelength of 193 nm and the calcium fluoride single crystal has a refractive index of 1.50 at the wavelength of 193 nm.
In the single crystal materials, there arises a problem of an occurrence of intrinsic birefringence (IBR). MgO and Al2O4 have IBR values of 70 nm/cm (extrapolated value) and 52 nm/cm (extrapolated value), which are considerably larger than that (3.4 nm/cm) of CaF2 (John H. Burnett, Simon G, Kaplan, Eric L. Shirley, Paul J. Tompkins, and James E. Webb, “High-Index Materials for 193 nm Immersion Lithography, Preprint—Proceedings of SPIE Vol. 5754—Optical Microlithography XVIII (2005)).
For this reason, development of a material causing no IBR is required. As an example of development of a high-refractive index optical member having a high transmittance in the ultraviolet region, an attempt is made to increase the refractive index by realizing permanent high density of quartz glass under application of a pressure (Phys. Chem. Glasses 10, 117 (1969)). However, a change in refractive index by the pressure application is small, so that the above described high-refractive index optical member has not been put into practical use.
In summary, it is difficult to increase the refractive index of the quartz glass, and the crystalline optical member such as the lutetium aluminum garnet single crystal causes the IBR. Further, when the above described optical members are used in an immersion exposure device as an optical device, the optical members have not been sufficient in terms of various characteristics.